Many current electronic communication systems use quadrature modulation schemes, which use in-phase (I) and quadrature (Q) signal components, and do not have constant envelopes. Examples of such communication systems are cellular radio telephone systems that use wideband code division multiple access (WCDMA), orthogonal frequency division multiple access (OFDMA), and their variants. Thus, part of the communicated information is encoded in the amplitude (envelope) of the transmitted signal and part is encoded in the phase of the transmitted signal.
To avoid distorting the communicated information, the power amplifier (PA) and various other components of the radio transmitter have to be linear, which is to say for example that the functional relationship between the output power of the PA and the input power of the PA is a straight line for all possible power levels. In addition, the phase shift of the input signal for example through the PA has to be constant for all possible power levels.
Departures from amplitude linearity and constant phase introduce distortion into the communicated signal, such as spectral broadening that can disturb adjacent channels. Amplitude/phase distortion (vector distortion) in the transmitter can also increase the bit error rate (BER) of the communication system, e.g., degrading the audio quality of a voice call or reducing the speed of an internet connection.
In general, the likelihood of proper performance can be increased by including in the transmitter a measurement receiver (MRX) that samples the transmitted signal and generates a compensation signal is fed back to the modulator, PA, and/or other transmitter components to correct the transmitter output signal. Such an arrangement 100 is depicted in FIG. 1, which shows an antenna 102, a coupler 104, an amplifier 106, a quadrature modulator 108, and an MRX 110. The amplifier 106 and modulator 108 can be considered the “transmit path” of the arrangement 100, which it will be understood typically includes oscillators and other components not shown. As seen in the figure, the MRX 110 samples the transmitted signal generated by the transmit path through the operation of the coupler 104 and provides a compensation signal to the modulator 108.
The MRX 110 can be used for several purposes, one of which is measurement, or more generally estimation, of the direct-current (d.c.) offset between I and Q components in the amplifier 106 and quadrature modulator 108. To achieve that purpose correctly, the I/O d.c. offset of the MRX itself typically must be negligible (ideally, it should be zero) or at least well known. Otherwise, the I/O d.c. offset of the transmit path will generally not be correctly estimated.
European Patent Application Publication No. EP 1 835 626 A1 by Ishikawa et al. describes a d.c. offset correction value estimating unit that estimates a d.c. offset correction value based on a transmit signal that is produced by a quadrature modulator. A signal level detecting unit detects the signal level of an input signal, a weight factor calculating unit computes a weight factor for the d.c. offset correction value in accordance with the signal level, and a weighting unit assigns a weight to the d.c. offset correction value in accordance with the weight factor. A d.c. offset in the transmit signal is compensated by using the thus weighted d.c. offset correction value.
U.S. Patent Application Publication No. US 2007/0092023 by Kang et al. describes a method for self-calibrating mismatch and d.c. offset in a mobile transceiver. The transceiver's transmitter is used as a signal generator and the transceiver's receiver is used to measure a response characteristic. A baseband processor calibrates the mismatch and the d.c. offset for the receiving and transmitting sides using a test signal received from the transmitter.
U.S. Pat. No. 7,266,359 to Chen et al. describes a method for removing d.c. interference from a signal received by a communication receiver that removes d.c. offsets induced by the receiver and the transmitter. The method includes removing an estimated d.c. offset from a received signal, correcting a frequency shift in the received signal, estimating a second d.c. offset signal induced by the transmitter, and removing the estimated second d.c. offset from the received signal. The receiver d.c. offset is estimated and removed before performing a timing carrier offset correction using Barker code manipulation to remove receiver d.c. offset and to sum all Barker chips after effectively multiplying Barker codes to correlate to a Barker sequence unaffected by the receiver d.c. offset signal.
U.S. Patent Application Publication No. US 2008/0063113 by Gao et al. describes a method of correcting d.c. offset errors in a transmitter having an OFDMA-based quadrature modulator. A compensator before the modulator compensates the d.c. offset and is updated with estimated d.c. offset values obtained by performing a discrete Fourier transform in the digital baseband domain while sending a pair of orthogonal test tones to the modulator's inputs.
U.S. Patent Application Publication No. US 2009/0041161 by Jian et al. describes a d.c. offset estimation in an OFDMA system that includes a carrier frequency offset estimator receiving an input signal and estimating a carrier frequency offset value, a symbol timing recovery unit providing a symbol boundary of the input signal, and a d.c. offset estimator estimating a d.c. offset value based on the input signal, the carrier frequency offset value, and the symbol boundary.
Despite those and other previous attempts, the problem of d.c. offset estimation remains difficult to solve, and the requirements on an MRX remain difficult to meet.